Pulse signal proportional control servosystem



C. E` HILLS April 24, 1962 PULSE SIGNAL PROPORTIONAL CONTROL SERVOSYSTEMFiled Jan. '7, 1954 5 Sheets-Sheet 1 C. E. HILLS April 24, 1962 PULSESIGNAL PROPORTIONAL CONTROL SERVOSYSTEM Filed Jan. 7, 1954 5Sheets-Sheet 2 5 Sheets-Sheet 3 April 24, 1962 c. E. HILLS PULSE SIGNALPRoPoRTIoNAL CONTROL sERvosYsTEM Filed Jan. fr, 1954 C. E. HILLS April24, 1962 PULSE SIGNAL PROPORTIONAL CONTROL SERVOSYSTEM Filed Jan. 7,1954 5 Sheets-Sheet 4 April 24, 1962 c. E. HILLS 3,031,603

PULSE SIGNAL PROPORTIONAL CONTROL SERVOSYSTEM Filed Jan. '7. 1954 5Sheets-Sheet 5 VL -I/7 Ml. Z! Ila/ H I -lv United States Patent3,031,603 PULSE SIGNAL PRGPORTI'ONAL CGNTRL SERVOSYSTEM Campbell E.Hills, Lewiston, N.Y., assigner to the United States of America asrepresented by the Secretary of the Air Force Filed dan. 7, 1954, Ser.No. 402,831 9 Claims. (Si. S18- 16) This invention relates to remoteproportional control systems of the command position type. It is theAgeneral object of the invention to provide a proportional control`system in which the position of an element can be accurately controlledfrom a remote point by a unidirectional transmission of energytherebetween. More specific objects of the invention are to provide acommand position remote control system in which the control informationis contained in they width or duration of pulses of energy transmittedat a constant repetition rate, and to provide such a system in which thepossible control functions include the continuous rotation of a shaftthrough any number of degrees in either direction.

in general, the control station comprises, in each control channel, anoscillator of frequency distinct from the other oscillator frequenciesand means for gating the oscillator to produce an alternating currentpulse having a duration proportional to the desired position in acontrol function. These pulses are then transmitted to the controlledstation over a suitable energy transmission link, such as radio or atransmission line, Where they are separated by tilters and the pulsesfrom each channel converted into a voltage proportional to theirduration. This voltage is applied to a suitable servo system whichbrings the controlled object to the position indicated by the voltageamplitude. In providing `for continuous shaft rotation in eitherdirection, the shaft rotation is divided into two 180 sectors with thearrangement that, for continuous rotation in one direction, the controlpulse in creases from its minimum Width to its maximum width in onesector and decreases from its maximum width to its minimum width in theother sector, thus avoiding the discontinuity in control pulse width andreversal of the direction of rotation that would otherwise occur uponreturning to the position. Where the output shaft is used to position anelectrical pick-olf, a simpler system is disclosed in which a singlepulse is used for 360 of rotation and the pick-off is short circuitedduring its rapid reverse rotation accompanying the abrupt change ofpulse width between its maximum and minimum values.

A more detailed description of the invention will be given in connectionwith the accompanying drawings which illustrate a proportional controlsystem in accordance with the invention designed for the speciiicpurpose of controlling the altitude and magnetic heading of an aircraftor missile. In the drawings FlG. 1 shows the control station;

. iilG. 2 illustrates the operation of the pulse width control circuit;

FIGS. 3, 4 and 5 show the controlled station;

FIG. 6 illustrates the control pulse width variation in the azimuthchannel;

FIG. 7 illustrates the operation of the autopilot adapter;

FlG. 8 is a schematic diagram of the autopilot adapter; and

iilGS. 9a and 9b show an alternative azimuth control system.

ln the altitude-azimuth control station shown in FIG. l, the operationof the altitude control channel Will be considered first. With altitudeswitch 1 in controller 2 in the On position, relay K1 is energizedclosing contacts K10. and Klc. Closure of contacts Kla grounds thecathode of tube 3 and initiates oscillation of the 2800 'ice c.p.s.oscillator circuit of which this tube forms a part. The oscillatoroutput, the amplitude of which may be adjusted by potentiometer 4, isapplied to one terminal of gate relay 5 the other terminal of which iscoupled to the grid of isolating amplifier 6. Therefore, whenever relay5 is closed audio energy is applied to amplifier 7 and thence totransmitter 8 where it modulates the radiated carrier frequency.

The operation of gate relay 5 is controlled by gate tube 9 which obtainsconstant operating voltages from gaseous voltage regulator tube 10.Condenser 11 and rectier 12 limit the negative peaks and back waveappearing across the relay coil. The voltage on the `grid of tube 9 isthe sum of the voltage at contact 13` of altitude control potentiometer14, the minimum and maximum values of which may be adjusted by contacts105 and 166, and the instantaneous value of the triangular wave appliedthrough contacts Klc from triangular wave generator 15. This generatorcomprises a continuous circular resistance element 16 having a constantdirect voltage applied across taps 17 and 18 which are exactly 180apart. The output is taken from contact 19 which is rotated at aconstant speed of l0 r.p.m. by motor 20.

The altitude control potentiometer 14 may be calibrated over an altituderange between predetermined minimum and maximum values. Contact 13,which may be set to any desired altitude within this range, adjusts thebias on the grid of tube 9 and thereby determines the portion of eachtriangular wave cycle during which the relay 5 is closed. This action isillustrated in FIG. 2. As the bias is made less negative the gridpotential exceeds the threshold value of the relay -for an increasinglygreater part of the triangular wave cycle and the gate length isincreased correspondingly. The output of tube 6 therefore consists of aseries of alternating current pulses having a repetition rate of 10 persecond and a length or duration proportional to the requested altitude.

The altitude signal from transmitter 8 is received and demodulated byreceiver 21, FIG. 3, located at the controlled station. The resultingalternating current pulses, identical to those in the output of tube 6,are applied through filter 22 to amplifier 23 and occur in amplifiedform at terminals A. The altitude channel of the controlled station isshown in FdG. 4. The signal from ampliiier 23, FlG. 3, is appliedthrough terminals A to altitude demodulator 24. The purpose of thedemodulator is to convert the applied series of alternating currentpulses into a direct voltage the amplitude of which is proportional tothe number of audio cycles contained in each pulse. This is accomplishedby first limiting the applied pulses to a constant amplitude in tube 25,the positive half-cycles being limited by grid conduction and thenegative half-cycles by anode current cut-off. Each of the major pulsesin the output of tube 25 therefore consists of a series of shorterpulses at the 2800 c.p.s. rate as shown in FIG. 4. Each of the shorterpulses causes a current to flow through condenser 26, diode 27-23 andresistor 2d during the existence of the pulse. The resulting incrementin the charge on condenser 26 is removed during the intervals betweenthe shorter Apulses by diode Sil-3l. Since a iixed amount of currentiiows through resistor 29 each time one of the shorter pulses isapplied, the average current through this resistor over a period of timelong in comparison to the major pulse repetition period of V19 second isproportional to the number of shorter pulses occurring in each majorpulse or, in other Words, to its Width w. Elements 32 through 37constitute a low pass lter to reject the high frequency components ofthe square wave of voltage across resistor 29, elements 37 and 34 alsobeing part of a twin 'l' filter, including also elements 38, 39, 40 and41, designed to reject the l0cycle repetition rate of the o) squarewave. The output of this filter, which is a direct voltage elproportional to the width w of the major pulses, is applied to the gridof tube 42 in altitude position servo amplifier 43.

Servo vamplifier 43 supplies alternating current energy to variablephase winding 414 of two-phase servomotor 45, the fixed phase winding loof which is energized through transformer 47 from ksource 48. The rotorof servomotor 45 drives Contact 4% of linear follow-up potentiometerthrough a suitable mechanical coupling 5l such as a shaft. The potentiale2 at contact d@ is applied to the grid of tube 52 in the servoamplifier for comparison with the command signal on the grid of tube 42.VOperation of the servo amplifier may be explained by first consideringthe potentials e1 and e2 on the grids of these 'tubes to be equal. lWith these two potentials equal the Aoutputs of tubes l2 and 52, whichare connected as cathode followers, are equal and the potentials on thegrids of tubes 53 and 54- are also equal. Alternating potential `fromsource i8 is applied to the anodes of tubes 53 and 54 in parallel. Sincethe grid potentials of these tubes are equal, the anode currents ofthese tubes are equal or substantially so, and the potentials of their4cathodes relative to ground are equal or can be made so by adjustmentof contact 55. With equal cathode potentials the voltage across theprimary of transformer 5d is zero and consequently there is no signal onthe grid of tube 57 and no output from phase inverter stage 57"-58.

The grids of tubes 59 and 'all receive equal and oppositely phasedVsignals from transformer 6l. Alternating potential from source 4S isapplied in equal amplitude 'and phase between the anodes and cathodes ofthese tubes. With zero signal from transformer 6l the currents throughresistors 62 and 63 are equal and equal negative potentials are appliedto the grids of tubes 64 and 65. The coupling networks between tubes59--60 and tubes olif-62S are identical T-type networks, one networkconsisting of elements 74-75--76-62 and the other consisting of elements''7-7E-79-63. These networks are designed to lgive a phase advance ofapproximately 55 at '4 cps. and serve to stabilize the servo system,allowing more gain without hunting or overshoot. A transformer isconnected to the output circuits of tubes da and 65 which has a winding6o controlling the saturation of the magnetic path between primarywinding 67 and secondary winding o3, and awinding 69 controlling thesaturation of the magnetic path between primary winding 70 and secondarywinding '71. Primary windings 67 and 'tl are equally energized fromsource 4S while secondary windings o?, and 7l are connected in seriesyWith opposed phases so that when the couplings between the primariesand secondaries are equal the output or the servo ramplifier is zero.These couplings are equal, or can be made so by adjustment of tap 72,when the potentials on the grids of tubes S9 'and oil` are equal.Therefore, when the follow-up potential e2 from contact 49 applied tothe grid of tube52 equals the command signal e1 on the grid of tube 42,Variable phase winding 4riis deenergized and the servomotor does notrun.

A difference in the potentials on the grids of tubes 42 and 52 producesan energization of winding 44 in proportion to the difference and havingone of two inverse` phases 'depending upon which of the two gridpotentials is Ithe greater. The direction of the resulting rotation ofkthe servomotor is such as to bring the voltage on the grid of tube 52into equality with that on the grid of tube 42. Assuming, for example,that the voltage on the grid of tube 42 exceeds that on the grid of tube52, an output will appear at the secondary of transformer 56 having amagnitude proportional to the difference and having a phase either equalto that of source 48 or opposite thereto depending upon the transformerconnections. lThis loutput ,is converted into a balanced output signalby phase inverter stage 57-53 so that the signals on the grids of tubes59 and 60 are of opposite phase and equal amplitude. Since iii-phasealternating potentials exist between the anodes and cathodes of thesetubes it follows that the grid and anode phases will be Ithe same in onetube and the opposite in the other. Anode conduction in the tubereceiving the in-phase voltages increases whereas that in the tubereceiving the oppositely phased `voltages decreases with the result thatconduction in one of tubes 64% and 65 increases while that in the othertube decreases. This causes an increased saturation and reduced transferof energy in the case of one set of primary and secondary windings ofthe output transformer and decreased saturation and increased energycoupling inthe case ofthe other set. Consequently a differential outputis produced from the series connected sccondaries d8 and 7l, the phaseof which is the same as that or the predominant secondary voltage. Thisoutput is applied to winding 44 of the servornotor, the arrangementbeing such that the resulting rotation of the motor moves the tap il@ ina more positive direction Vuntil the voltage on the grid of tube 52 isequal to that on the grid of tube 42. Condenser 73 serves with theinductan'ces in the circuit to establish a 90 phase difierence betweenthe current in winding 44 and that in winding The operation of thecircuit when the command signal is less than the voltage on the grid oftube 52 is the same as that described above except that the phaseconditions are reversed and the servomotor rotates in the oppositedirection to move contact 49 in a negative direction until equality ofthe two voltages is achieved.

The final result or the above control system is to position shaft 5l ata point in its range of angular movement corresponding to the positionof Contact i3 of altitude control potentiometer le (FiG. l) within itsrange of movement. 7he altitude of the aircraft is controlled by shaft5l in the following manner: Shaft 5l is coupled to secondary winding @ilof an altitude pick-ofi transformer the primary winding dl of which isenergized from source 43 and positioned by aneroid unit h2.. Altitudecontrol potentiometer i4 operates through the above described remotecontrol system and shaft 5l to position winding Sil at a pointcorresponding to the requested altitude. it' the aircraft is not at therequested altitude a voltage is induced in winding h which is eitherirl-phase or of opposite phase with respect to source [i8 depending uponwhether the actual altitude is greater or less than the requestedaltitude. This signal, appearing at terminal B, is applied to thealtitude control circuit of the aircraft and causes it to ascend ordescend, as the case might e, seeking the requested altitude. As thealtitude changes winding tlf. is moved relative to winding Sti byaneroid element li?. in such direction as to reduce the coupling betweenthe two windings. When the aircraft has attained the requested altitudethis coupling is zero so that the signal at terminal B is likewise zeroand the aircraft resumes level flight. Any tendency for the aircraft todeviate from the requested altitude results in 'a counteracting signalat this terminal. if potentiometer lidi of FiG. l is moved to a newposition winding is moved to a corresponding new position and theaircraft seeks the corresponding new altitude by the processdescribedabove. A small motor 83 controlled by circuit /lmay bedifferentially geared to winding Sti for adjusting its position relativeto that of potentiometer Contact t9 to permit adjustment for changes inbarometric pressure.

The altitude hold control 85 in FlG. 4 keeps the altitude servopositioned during periods when the transmitted signal is off ortemporarily lost dus to fading or when the channel is turned olf. 'Ihehold control is necessary since the instant the received signal ceasesthe servo amplifier input voltage on the grid of tube 42 begins to fall.Any change in input signal calls for a new servo position and instantlythe servomotor will run to seelt the new position. The faster the motor,the quicker are closed and KZb are open.

The operation of control signal is being received, is sufficient toovercome the bias on this tube due to the return of its cathode to asource of positive potential. During normal operation, therefore, relayK2 is energized so that contacts KZa. A negative bias derived from therectification of alternating Voltage from source 43 is applied to thegrid of tube 93 and, together with the self bias developed acrossresistor 94, is sufficient to keep relay K3 deenergized when contacts Kbare open. With relay K3 deenergized, the cathodes of tubes 64 and 65 areconnected to ground through contacts Kita and KZa, and voltage fromsource 'd8 is applied to primaries 67 and 7) through contacts K3b.

Following each leading edge pulse applied to tube 89 condenser 9@discharges almost to the point of release of relay K2. Therefore if thenext leading edge pulse is v absent the discharge will continue andrelay K2 will be deenergized. The opening of contacts KZa disconnectsthe cathodes of tubes 64 and 65 from ground, and the closing of contactsK2b grounds the grid of tube 93. The latter action raises the potentialof the grid of this tube and causes suicient conduction to energizerelay K3. The opening of contacts Kb removes the A.C. energization fromprimaries 67 and itl and stops servomotor 45.

Upon return of the altitude control signal relay K2 is reenergized bythe first leading edge pulse. This opens contacts KZb which allows thegrid of 93 to become more negative at a rate determined by the size ofcondenser 95. The required rate is determined by the time constant ofthe filter in the output circuit of demodulator 24 and is made such thatthe servo amplifier input signal on the grid of tube 42 reaches a steadycondition before deenergization of relay K3. With the deenergization ofthis relay the cathodes of tubes 64- and 65 are again connected toground through contacts KSa and K2a and the primaries 67 and 7i) areagain energized from source 48 through contacts Kb for normal operationof the servo system.

The principal difference between the altitude control channel and theazimuth control channel is that the latter is designed to permit acontinuouscycling of the control function in either direction. Thealtitude control function could be varied only back and forth betweenminimum and maximum limits. The structural dierences of the azimuthchannel as compared with the altitude channel are to be found in thecontroller of FlG. l and the azimuth indicator unit of FIG. 5. Withreference to these figures, azimuth control dial 1M) may be rotatedcontinuously in either direction and be followed by shaft 101 of FIG. 5.The manner in which this is accomplished will be explained in thefollowing detailed escription of the azimuth control channel:

rEhe azimuth channel is placed in operation by placing switch 192 incontroller 2 in the On position which will ground relay K4 provided thealtitude switch 1 is in the Off position. The two channels areinterlocked by this arrangement of switches to permit operation of onlyone channel at a time with precedence given to the altitude channel.Energization of relay K4 places in operation the azimuth signalgenerator circuit103 which is identical in all respects save thefrequency to altitude signal generator circuit 104, the operation ofwhich has already been explained. The output of this circuit, as in thecase of circuit 1M, is a series of alternating current pulses the widthsor durations of which represent the position of dial 100.

In the azimuth control system the 360 of azimuth are divided into aNorth sector extending, in a clockwise direction, from 284.5 to 104.5and a South sector extending from 104.5" to 284.5". The width of thealtitude control pulse produced by circuit 103 is made to vary,considering clockwise rotation, from a minimum value at 284.5 to amaximum value at l04.5 and back to a minimum value at 284.5 asillustrated in FIG. 6.

The mechanism for accomplishing the above is shown in FIG. l. ial drivesshaft 107 on which are mounted cam 1% and the continuously rotatablearms of potentiometers 109 and 110. The resistance elements of thepotentiometers have the same regular positions while the arms on shaft107 are separated by 180. Cani 103 is positioned relative to dial 100 sothat switch 111 is closed in the South sector, or from 104.5 to 284.5",and open in the North sector, or from 284.5 to 104.5", consideringclockwise rotation. Further, the arms of potentiometers 169 and 110 arepositioned on shaft 197 so that the arm of potentiometer 109 contactsits resistance element throughout the North sector and the arm ofpotentiometer 11d contacts its resistance element throughout the Southsector.

Potentiometers 109 and 11G control the width of the azimuth controlpulse Igenerated in circuit 163 by controlling the bias on gate tube 9'in the same manner as pulse width control was obtained in circuit 194 ofthe altitude channel, already explained. Cam actuated switch 111 andrelay K5 operate to energize and connect potentiorm eter 109 to the biascontrol circuit when the desired azimath angle is in the North sectorand to energize and connect potentiometer 11i) when the angle is in theSouth sector. Accordingly, as will be seen in FIG. 1, relay K5 isdeenergized in the North sector so that potentiometer 109 is energizedthrough contacts Kf and its contact arm is connected to the grid of tube9 through contacts KSd. In the South sector, relay K5 is energized andoperative connections are made to potentiometer 11i) through contactsKSe and KSC. Energization for the potentiometers is obtained fromcontacts and 166 which are provided for adjusting the maximum andminimum voltages applied to the potentiometers, so as to control themaximum and minimum widths of the generated azimuth control pulses.

As shown in FIG. 6 the pulse width of the transmitted azimuth controlsignal is a direct function of the azimuth angle in the North sector andan inverse function thereof in the South sector. 1t is thereforenecessary for the controlled station to know when transitions betweenNorth and South sectors occur in order to be able to adjust its controlcharacteristic accordingly. The indication of these transitions is thefunction of relays Kd and K7. When a transition is made from the Southto the North sector switch 111 opens and condenser 112 charges throughthe winding of relay K6 and contacts KSb. For the short period duringwhich the charging current exceeds the threshold current of the relaycontacts K6b are closed and contacts Kotz are open. lf either theazimuth or the altitude channel is operative at the time, the opening ofcontacts Ka disables it by breaking the energizing circuit of relay K4or K1, since it is desirable that sector switching be given precedenceover other control functions. Closure of contacts K6b grounds thecathode of tube 113 in 1600 cps. North sector oscillator 114, causingthe generation of a 1600 c.p.s. pulse of short duration which is appliedto the transmitter and which serves as the signal indicating theSouth-to-North sector transition. For the North-to-South transition,switch 111 closes actuating K6 and allowing condenser 115 to chargethrough the K7 winding and contacts KSa. The resulting momentaryoperation of K7 disables the altitude or azimuth channel through theaction of contacts K7a and renders 1050 c.p.s. South sector oscillator116 operative through the closure of contacts lib. he resulting short1G50 c.p.s. pulse is applied to the transmitter and constitutes thesignal indicating the North-to-South transition.

At the controlled station the azimuth signals are channelled into theirappropriate circuits by filters 117, 118 and 119, FlG. 3. The signalsappearing at terminals C, D and E are identical to those at the outputsof genrator 163 and oscillators 114 and 116, respectively, in

FlG. l. The azimuth channel of the controlled station is shown in FIG.5. lts essential function is to rotate shaft M1 in step with dial 1li@of FIG. l through any number of revolutions of the dial in eitherdirection. This is accomplished in the following manner:

The signal at terminals C, which is a' series of alternating currentpulses the widths of which indicate the position of dial ft, is appliedto demodulator Zd. This demodulatcr is identical to the previouslyexplained demodulator 24 of the altitude channel and operates to convertthe applied pulses into a direct voltage e1 the amplitude of which -isindicative of the position of dial 1li@ or in other words the requestedazimuth. This voltage and a follow-up voltage e'2 are applied to azimuthposition servo amplifier 43 which is identical to previously explainedamplifier 4.3 in the altitude channel. An alternating voltage s1 isproduced by the servo amplifier that is proportional to the ditlerencebetween el andl e2, being zero when these two voltages are equal. Thevoltage s1 is derived from source d8 (FIG. 4) and has a phase eitherleading or lagging that of this source by 90 depending upon which of thetwo voltages @f1 and e'2 is the greater. The output of the servoamplifier is applied to variable phase winding 12d of two-phaseservomotor 121. Winding 122 of the servomotor is also energized fromsource 48 (FIG. 4), the phase being the same as or opposite to that ofthe source depending upon the sector, as will be seen later.

Servornotor 121 drives shaft lul which carries, with 180 spacing, thecontact arms of North sector potentiometer lull and South sectorpotentiometer 110. These are continuously rotatable potentiometerssimilar to corresponding potentiometers 169 and 11th in the controllerof FiG. V1. Relays K3 and K9 respond to the North sector and Southsector signals at terminals D and E to connect the proper potentiometerinto the circuit and to supply the proper phase to winding 122. When thecontrol dial 10d of FIG. l passes from the South to the North sector ashort 1600 c.p.s. pulse appears at terminal D. Tube 123 is normallybiased sufficiently to reduce the anode current below the threshold ofrelay K9. The signal at terminal D therefore increases the anode currentand energizes K9. A latching mechanism is provided between KS and K9 sothat `when either is momentarily energized it isrlatched in its operatedposition and the other relay is released from its operated position.Therefore the effect of the signal at D is to close contacts K9@ Kde,Kuby and KS4. As a result North sector potentiometer MP9 is energizedthrough contacts KSZ) and its contact arm is connected to the servoamplifier through contacts Kde. Further, the phase of the voltageapplied to winding 122 through contacts K9a and K9c is such that shaft101 will be driven clockwise when e'2 is less than el andcounterclockwise when greater than ell.

When control dial ltltl passes from the North to the South sector ashort 1050 c.p.s. pulse appears at terminals E and the grid of tube 124.This pulse actuates relay KS and releases relay K9. As a result, Southsector potentiometer 11d is energized and its contact arm connected tothe servo amplifier through now closed contacts lido and KSC,respectively. Also, closure of contacts Klb and KM reverses the phase ofthe Voltage applied to winding 122 so that shaft 101 is now drivenclockwise when e2 is greater than el and counterclockwise when e'2 isless than @'1. Continuous rotation of shaft o lill ineither direction inresponse to similar rotation of dial lull is therefore possible.

The heading of the aircraft is controlled by the Voltage induced inwinding 125, the rotor winding of a synchro, as a result of its couplingto the stator windings 126. The stator windings are energized withalternating current from a magnetic compass, the current being derivedultimately from source 48, so that the ilux produced by the windingsserves as a magnetic North reference. Winding is driven by shaft lul andits angular position thereon is such that with shaft 101 in the 0azimuth position, i.e. with azimuth control dial ltlil at 0, and theair-V craft heading to magnetic North, zero voltage is induced inwinding 125. lf the aircraft deviates from this heading a voltage isinduced in winding 125 which has the same or the opposite phase withrespect to the phase of source 48 depending upon the` direction ofdeviation. This signal, when applied to the turn control circuit of theaircraft, opposes the deviation and maintains the heading at magneticNorth. Any new heading of the aircraft may be established by settingazimuth control dial lili? in the control station to the desiredheading. This Will result in shaft 101 and Winding 125 assuming aposition corresponding to the requested heading. The voltage induced inwinding 125 will then change the heading of the aircraft in thedirection of the requested heading which will cause the tux produced bywindings 126 to rotate with respect to these `windings and the windingM5. When the requested heading has been reached the flux will be in suchdirection with respect to winding M5 as to induce zero voltage thereinand the aircraft will be automatically held at this heading by theprocess described above until a newV heading is requested.

Azimuth hold circuit d5 is similar in all respects to the altitude holdcircuit 85 of FIG. 4, the operation of which was explained in connectionwith the altitude channel so that further discussion is not necessary.

All corrections for azimuth are made by signals introduced into the turncircuit of the autopilot. ln normal level lght the roll and pitch axesof the aircraft are kept parallel to the earths surface by automaticerection of the vertical gyro in the autopilot. If, during turns, thiserection system were not disabled false erection would take placecausing the plane to leave the turn out of level. The erection cut-offcircuit 127 prevents this by disabling the erection system whenever asignal appears at the output of winding 125'. This signal is applied tofull wave rectifier 12.3 the output of which is the same for eitherphase of the input signal. After amplification this wave is applied tocontrol electrode 129-of gaseous tube 13d which has relay K9 in itsanode circuit. The anode circuit is energized from source 43 (FIG. 4)and since the alternating voltage on electrode 129 has twice thefrequency of that on the anode of tube 3?, the grid is driven positiveduring each half-cycle of the voltage applied to the anode. Tube Btl maytherefore be fired by a signal from winding 125 of either phase. RelayK9, when energized, operates a relay in the autopilot disabling theerection system. Potentiometer 131 controls the sensitivity of thecut-off circuit and should be adjusted so that relay K9 operates for allerror signals f equivalent to 3 or more; this setting, however, may bevaried to Asuit conditions.

The signal from winding 8d of the altitude control channel (FIG. 4) andthe similar signal from winding 125 of the azimuth control channel (FIG.5*) are not suitable for direct application to an autopilot.r The reasonfor this may be seen in FIG. 7 as applied to the azimuth channel. Curve131 represents the output voltage characteristic of the synchro in theazimuth indicator unit of FIG. 5. If this signal were fed directly tothe aileron channel of the autopilot a ninety degree rotation of thesynchro would be required to achieve 30 bank of the aircraft, and incorrecting lesser errors of heading the aircraft would bankcorrespondingly less. Course corrections therefore would be veryinsensitive. If the sensi- Itivity were increased so that 10 rotation ofthe synchro produced the output required for a 30 bank, as illustratedby curve 132, then further rotation of the synchro would increase thevoltage to a point beyond the circuit limits of the autopilot. A limitercould be used to limit the output voltage beyond a certain value.However, the output would then contain a high percentage of harmoniesand would not be satisfactory for injection into an autopilot. A similarsituation exists with respect to the output of Winding 80 in thealtitude channel of PEG. 4, wherein, .if the signal at terminal B wereapplied directly to the elevator channel ofthe autopilot, a 90 rotationof winding 80 would be required to attain full elevator.

Autopilot adapters 133 in the output circuits of the altitude andazimuth channels modify the above signals to provide suitable signalsfor the autopilot. The schematic diagram of this device is shown in FIG.S. Referring to this figure, tubes 134A and 134B serve as an amplifier,discriminator and limiter. The signal from winding 80 or 125 is appliedto the grids of these tubes in parallel.` The anode circuits areenergized with oppositely phased alternating current from source 43 byway of transformer 135. Since the signal on the grids has either thesame or the opposite phase relative to source 48 its phase relative tothe anode phase in each of tubes 134A and B will be the same or oppositedepending upon the phase of the input error signal. In the absence of asignal, the anode currents in tubes 134A and 134B are the same and thenegative direct potentials developed across resistors 136 and 137 arethe same. These potentials are applied through low-pass filters to thegrids of tubes 138A and 138B. The anode currents of these tubes iiowthrough saturation control windings of saturable reactors 1.39 and 140which form twoyadjacent arms of an alternating current bridge, the othertwo arms of which are formed by the two halves of resistor 141. Thebridge is energized from source 48 across opposite points 142 and 143and its output is derived from opposite points 144. and 145. Y

With zero error signal the anode currents of tubes 136A land 134B areequal and, therefore, thevanode currents of tubes 138A and 138B are alsoequal. For this condition there is zero output from the bridge; or theoutput may be made to equal zero by slight adjustment of tap 145. Whenan error signal is aplied to the grids of tubes 134A and 134B conductionin the tube in which the anode and grid are in phase is increased andconduction in the tube in which the anode and grid are out of phase isdecreased. Consequently the grid of one of tubes 135A and 132123 is madeless negative and the grid of the other is made more negative. In theformer case, as the grid becomes less negative the anode currentincreases, eventually saturating inductor 139, the tube, or both, sothat no further decrease in its reactance takes place. ln the lattercase,

the increasingly negative grid reduces the anode current untileventually cut-oit is reached and no further increase in the reactanceof inductor 140 takes place. The output of the bridge at this point is amaximum and no further increase in error signal input will affect it.The gain of tubes 138A and 138B is sufciently high that maximum bridgeoutput may be attained at a comparatively small error signal, the exactvalue of which may be adusted by potentiometer 144. In the case of theazimuth channel the circuit may be adjusted to produce maximum output atan error signal equivalent to 10 as shown in FIG. 7. The output remainsat the maximum value as long as the error signal exceeds the 10 value.For a decreasing error signal, conduction is resumed in one of tubes138A and 138B and the'condition of saturation is relieved in the otherat the 10 value. The output of the bridge then decreases to zero withthe error signal.

FIGS. 9a vand 9b show an alternative azimuth control channel whichsupplies an electrical output which, for

practical purposes, is the same as the electrical output of winding inFIG. 5. This is accomplished without the necessity for a continuous 360rotation of the controlled shaft which considerably simplifies thecontrol channel as compared with that shown in FIGS. 1 and 5.

FIG. 9a illustrates the azimuth channel portion of the control station.The azimuth portion of the controller 2 requires only a single controlpotentiometer as compared with the two required in the system of FIG. l.This potentiometer, which is set in accordance with the desired azimuthand supplies a variable bias to the grid of gate tube 9 in azimuthsignal generator 103, may have any desired angular extent but preferablyhas an extent as near to 360 as practical construction will permit. Thesignal generator 103, as well as triangular wave generator 15, amplifier7 and transmitter 3, are identical to the corresponding units of FiG. 1which have already been ex- Y plained. When potentiometer 150 is at the0 position a ly to open the gate relay of the azimuth signal generator.

Therefore crossing the gap in the clockwise direction causes the outputof generator 103 to jump from maximum signal width to zero signal tominimum signal width and crossing in the counterclockwise directioncauses the output to jump 4trom minimum signal width to zero signal tomaximum signal width.

The azimuth channel at the controlled station is shown in FIG. 9b.Receiver 21, lter 117, demodulator 24', servo amplifier 43' and holdcontrol 85' are identical to the corresponding units of FIGS. 3 and 5.The voltage el is proportional to the duration of the received pulses.Follow-up voltage e2 is indicative of the position of shaft Y101'.Voltage s' is an alternating voltage proportional to the differenceel-eg and has one of two inverse phases depending upon the sign of thisdiierence. Voltage s is applied to variable phase winding 120 ofservomotor 121, the xed phase winding 122 of which derives itsenergization for the same source as that Ifrom which s is derived.

Only a single follow-up potentiometer 151 is required. Thispotentiometer, driven by shaft 101 is not arranged for continuousrotation but has stops at its 0 and 360 limits. Shaft 152, which carrieswinding 125, is coupled to shaft 101 through overdrive gear 153. Thearrangement is such that rotation of the Wiper of potentiometer 151 fromits 0 position to its 360 position, which required less than a completerevolutionof shaft 101', produces nevertheless a complete revolution o-fshaft 152 and winding 1215. v

If potentiometer 150 (FIG. 9a) is set to a desired azimuth,potentiometer 151 is rotated to a position corresponding to the sameazimuth by the same process as already described in connection with thealtitude and azimuth channels of FIGS. l and 5. However, assume azimuthcontrol potentiometer 150 to be set at 355 and that it is desired tochange the heading of the aircraft to 5 by rotating the potentiometerclockwise to the 5 position. Potentiometer 151 will lfollowpotentiometer 150 to the 360 position, but as the wiper of,potentiometer 151 enters the gap the voltage e'l will fall to zero andpotentiometer 151 will run counterclockwise in an effort to bring e2into equality with e'l. This results in counterclockwise rotation to the0 position where e2 has its minimum value. The wiper of potentiometer151 will then follow the Wiper of potentiometer 150 clockwise to the 5position. VConsidering the inverse of the above example, in which theheading is changed from 5 to 355 by counterclockwise rotation ofpotentiometer 150, a similar action occurs. Potentiometer 151 followspotentiometer 150 to ansiosa l l the position. When the wiper ofpotentiometer l50 enters the gap el falls from its minimum` value tozero. Since potentiometer iSl is stopped from further counterclocltwiserotation no action takes place until the wiper of potentiometer' d50 hascrossed the gap and is at the 360 point. At this point, el jumps to itsmaximum value and potentiometer tdi races in the clockwise direction tobring e2 into equality with el, bringing it to its 360 point.Fotentiometer 151 then follows potentiometer T50 counterclocltwise tothe 355 point.

From the above it is seen that passage of the wiper of potentiometer 50between the ends of its resistance element causes the wiper ofpotentiometer lidi to take the long way `around to the opposite end ofits resistance element, ln order to prevent the voltage induced insynchro winding ftZ by this reverse rotation from being applied to theautopilot a differential relay Ki@ is provided for shortiug theterminals of this winding whenever there is a predetermined differencebetween voltages e1 and 0'2. One winding of the relay is located in theanode circuit of tube ld, which has e'z applied to its grid, and theother winding is located in the anode circuit of tube lSS, which has @'1applied to its grid. With e1=e2 the currents in the two tubes may beequalized by potentiometers liso and l5? so that the rnagnetomotiveforce of one coil cancels that ofthe other and the relay contacts areopen.

Since the sensitivity of the servosystem is high and its operationalmost instantaneous, the maximum difference betwen el and e2 duringnormal operation is very small. The signal amplitudes on the grids oftubes lidi-i and i155 are adjusted by potentiometers iti-6 and E57 sothat the differences in e1 and e'2 occurring during normal opera-l tionwill not actuate the relay. However, when the wiper of controlpotentiometer i150 passes across the gap be* tween the ends of theresistance element a large diterence betwen el and e2 occurs and therelay contacts are closed. For example, for clockwise passage, when thepotentin ometers are at their 360 points e'l and e2 are equal and havetheir maximum Values. Assuming the heading of the aircraft to havereached 360, the output of winding 125 is Zero. lf ,the wiper ofpotentiometer 150 is now moved across the gap e1 rst drops to Zero andthen rises to its minimum value when the wiper contacts the 0 end of theresistance strip. Since, with the wiper of potentiometer lSLl at the 360point, eg has its maximum value a large difference exists between e'2and e1 and the conn tacts of relay K10 close keeping the output ofwinding `h2o' at Zero. As the wiper of potentiometer lll-l rotatescounterclockwise ez decreases becoming equal to e'1 when the wiperreaches the 0 point. The relay contacts now open but the output ofwinding 125 remains zero since it is in the same position as whenthecontacts closed. The electrical output of the winding 125 is thereforethe same as it would have been had a continuous rotation in one direcEtion been possible. A similar action occurs for counterclockwise passagethrough the gap. ln this case e'1 goes to its maximum value when thewiper of potentiometer 150 reaches the 360 point and potentiometer lflrotates clockwise to bring ez into equality therewith.

I claim:

1. A proportional control system comprising means for producing a seriesof pulses of constant repetition rate; means for varying the duration ofsaid pulses between minimum and maximum limits in accordance with thevariation of a quantity between minimum and maximum limits so that theratio ofthe diierences between the pulse duration and its limits equalsthe ratio of the differences between said quantity and its limits; meansfor transmitting said pulses to a remote location; means at said remotelocation for varying a second quantity between minimum and maximumlimits; and means responsive to said pulses and acting on said quantityvarying means for maintaining the ratio of the differences between saidsecl2 ond quantity and its limits equal (to the ratio of the dif`ferences between said pulse duration and its limits.

"2. A proportional control system comprising means for producing a.series of pulses of constant repetition rate; a

5 control shaft; means for varying the duration of said pulses betweenminimum and maximum limits in accordance with the rotation of said shaftbetween angular position limits so that the ratio of the ditlerencesbetween the pulse duration and its limits equals the ratio of theangular differences between the angular position of said shaft and itslimits; means for `transmitting sm'd pulses to a remote location; acontrolled shaft at said remote location; means for rotating saidcontrolled shaft between angular position limits; and means responsiveto said pulses and acting on said controlled shaft rotating means formaintaining the ratio of the angular differences bey tween the angularposition of said controlled shaft and its limits equal to the ratio ofthe differences between said pulse duration and its limits.

3. A proportional control system comprising means for producing a seriesof pulses of constant repetition rate; a controlV shaft; means forvarying the duration of said pulses between minimum and maximum limitsin accordance with the rotation of said shaft between angular positionlimits so that the ratio of the differences between the pulse durationand its limits equals the ratio of the angular differences between theangular position of said shaft and its limits; means for transmittingsaid pulses to a remote location; means for converting said pulses intoa command voltage, having minimum and maximum amplitude limits, suchthat the ratio of the differences between said command voltage and itslimits equals the ratio of the diierences between the durationl of saidpulses and its limits; a controlled shaft at said remote location; aservomotor for rotating said controlled shaft between angular positionlimits; means coupled to said shaft for producing `a follow-up voltagehaving the same minimum and maximum limits as said command voltage and,for any angular position of the shaft between the shaft limits, havingsuch value that the ratio of the differences between its value and itslimits equals the ratio of the angular differences between the shaftposition and its limits; and means responsive to the ditte-rence betweensaid command and follow-up voltages and controlling the energization anddirection of rotation of said servomotor for bringing said follow-upvoltage into equality with said command voltage.

4. Apparatus as claimed in claim 3 in which means are provided fordeenergizing said servomotor in the absence of said pulses.

5. .A proportional control system comprising means for producing aseries of pulses of constant repetition rate; a control shaftv rotatablethrough any number of degrees in either direction; means dividing acomplete rotation of said shaft into two sectors; means for linearlyvarying the duration of said pulses ybetween minimum and maximum valuesas a result of rotating said shaft through either of said sectors7 saidvariations in the. two sectors being inversely related forunidirectional rotation of the shaft through the tWo sectors; means forgenerating a and its limits; a controlledl shaft at said remote locationcapable of being rotated through any number of degrees in eitherdirection; means coupled to said controlled shaft for dividing therotation of said shaft into two 180 sectors and for producing afollow-up voltage that varies linearly between minimum and maximumvalues as a 13 result of rotating said controlled shaft through eitherof said sectors, the variations in said two sectors being inverselyrelated for unidirectional rotation of the shaft through the twosectors; a servo system mechanically coupled to said controlled shaftand responsive to a difference in said command and follow-up voltages torotate said shaft in a direction determined by the polarity of saiddifference voltage and proper for bringing said followup voltage intoequality with said command voltage, said servo system comprising meansacting upon receipt of either of said sector transition signals toinvert the relationship between the polarity of said difference voltageand said direction of rotation.

6. Apparatus as claimed in claim in which means are Y provided fordisabling said servo system in the absence of `said pulses at saidremote location.

7. A proportional control system comprising means for producing a seriesof pulses of `constant repetition rate; a control shaft rotatable ineither direction; means actuated by said shaft for varying the durationof said pulses linearly between minimum and maximum limits in less thanone revolution of said shaft, said duration changing abruptly from onelimit to the other in the remainderA of the revolution; means fortransmitting said pulses to a remote location; means for converting saidpulses into a command voltage, having minimum and maximum amplitudelimits, such that the ratio of the differences between said commandvoltage and its limits equals the ratio of the differences between theduration of said pulses and its limits; a controlled shaft at saidremote location rotatable lonly between limits separated by less than360; means actuated by said shaft for generatng a follow-up Voltage,having the same minimum and maximum amplitude limits as said commandvoltage,

such that the ratio of the ditferencesbetween said follow-up voltage andits limits equals the ratio of the angular differences between theposition of said controlled shaft and its limits; a servo systemmechanically coupled to said controlled shaft and responsive to adifference in said command and followup voltages to rotate said shaft ina direction determined by the polarity of said difference voltage andproper for bringing said follow-up voltage into equality with saidcommand voltage; means establishing an alternating magnetic field; anoutput winding rotatable in said iield; mechanical coupling meansbetween said controlled shaft and said winding for producing a completerotation of said winding for each rotation of said controlled shaftbetween its limits; and means responsive to said difference voltage forreducing the output of said winding to zero whenever the magnitude ofsaid diference voltage exceeds a predetermined value.

8. Apparatus as claimed in claim 7 in which means are provided fordisabling said servo system in the absence of said pulses at said remotelocation.

9. A signal generator for producing a series of alternating currentpulses of constant repetition rate, said. generator comprising: aconstant frequency oscillator; an output circuit; an electron tubehaving an anode, a cathode, a control grid and an anode circuitconnected between its anode and cathode; means for generating asymmetrical triangular Wave of voltage; means for generating a biasvoltage and for varying said voltage between pre determined limits;means for applying said variable bias voltage and said triangular wavevoltage in series between said grid and cathode; and means for forming aconnection between said oscillator and said output circuit when thecurrent in said anode circuit is in excess of a predetermined value.

Cox Dec. 2, 1941 De Ganahl Jan. 29, 1946

